The invention relates to a method according to the precharacterizing clause of claim 1, correspondingly designed devices for carrying out this method and receptacles for heating meals by this method, it being possible to carry out a wide variety of application areas in the catering industry, such as for example the holding of banquets, serving of meals in public establishments, such as hospitals and care homes in particular, and also the serving of meals in vehicles, in particular on aircraft.
The invention is based on the method and device for heating food described in DE 27 11 088, in which the heating of the food located in closed containers takes place by means of induction heating of conducting elements in the containers.
In the case of the solution described there, the use of special food receptacles is generally necessary, as is also the case elsewhere for the inductive heating of food. The use of special kitchenware or, as likewise specified in DE 27 11 088, specially fitted-out containers or trays, the increased cost aspect of which is not generally justified, has also generally only been accepted rarely by users up until now.
Furthermore, the heating which can be achieved by induced eddy currents takes place in the known systems exclusively by means of heat conduction, convection and radiation, which, in particular in the case of convection and radiation, is hindered by the medium surrounding the food to be heated, air, so that increased time spent and reduced efficiency have to be accepted.
A further disadvantageous aspect to be noted in the case of this known solution is that it is necessary for the effectiveness of this method that the distance between the induction coils and the metallic parts actually to be heated of the containers which are intended for accommodating the food and are to be heated and their positioning are set optimally. For this purpose, a complicated mechanism is proposed there, with which the elements accommodating the induction heating coils can be set appropriately before the heating operation, at least in their height.
In particular on aircraft, it is customary to provide conventional ovens which are operated by resistance heating and with circulating air for the heating of meals. The relatively low efficiency of these known ovens results in a great expenditure of time and energy being required, which has extremely disadvantageous effects in this sector in particular. The ovens, normally standardized in their dimensions, must be manually loaded with the containers, which may also include conventional tableware, in a relatively laborious way and be heated appropriately to the desired temperature over a time period of about 20 minutes and then removed again individually, assigned individually to trays and the fully loaded trays introduced into insulating carts and transported by them to the individual passengers, where they are handed out.
Since the various catering companies also use different receptacles of different materials and in different dimensions, there may be further increased energy consumption and not optimum utilization of the space available in the ovens used there.
It is therefore the object or the invention to specify a possible way of heating prepared meals in a short time, to the extent that they are suitable for consumption, with low expenditure, in a highly effective manner and with good quality.
This object is achieved according to the invention by the features of patent claim 1. Advantageous refinements and developments of the invention emerge from the features contained in the subordinate claims.
In the invention, it is important in particular that the heating of the items of food or meals, if at all, by convection, induction or heat radiation is achieved at least also additionally by hot steam. In this way, not only can the effectiveness, i.e. the efficiency, be improved and the heating time reduced, but a regeneration of the prepared meals can also be achieved, having positive effects not only in terms of improving the esthetic appearance but also in terms of flavor.
For this purpose, one possible way of proceeding is that the steam is generated directly in a device with which the meals are to be heated or externally generated steam is introduced in an apportioned manner into such a device and, in the latter case, the temperature is maintained at an adequate level with the aid of induction, so that condensation of the steam during the heating phase can be avoided. Another possible way is that the steam is generated directly in the device during the heating, the temperature being chosen in both cases such that hot steam can be used for heating the meals.
In the most favorable case, the steam can be generated with the aid of the induction heating coils, so that it is possible to dispense with other heating systems or elements.
In a favorable way, the receptacles in which the meals to be heated are contained are placed into a corresponding device and positioned with respect to the induction heating coils, it being possible for particularly favorable induction heating coils for heat from above and below to be arranged above and below such a receptacle which contains meals to be heated.
Since, according to the invention, virtually any receptacles are to be used, in other words it is not necessary to resort to special kitchenware, it is favorable the eddy currents in electrically conducting materials (preferably ferro- or paramagnetic), which in the form of foils, foil parts or metal sheets favorably constitute part of such a heating device or of push-in containers, which will be discussed in more detail later.
In this case, the metallic sheet-like elements generally to be used in particular for the heat from above may be of a slotted or plastic-coated form, allowing an automatic regulation of power, and accordingly also the temperature, an increase in the corrosion resistance and/or heat emitting capacity.
The sheet-like elements in which the required heat for the heat from below is to be generated by induction may also be advantageously used for the generation of the steam. In this case, these parts may be designed in specific regions in such a way that they can take the water for the steam generation required for the heating and regeneration of the meals and may be designed for example in a channel-like manner and, particularly advantageously, in an annular manner around the respective receptacle.
Such foils or metal sheets may be surface-treated, in order on the one hand to improve their heat emitting capacity and on the other hand to form a corrosion protection.
In this case, the foils or metal sheets may be arranged such that they are individually assigned to an induction heating coil, or else one such element is assigned to a plurality of such coils.
There is also the possibility of using an additional induction in the receptacles containing the meals, in which such a receptacle is enclosed by a ring of a material suitable for this purpose, which can concentrate the field in the direction of the receptacle base.
It must be noted that the sheet-like element responsible for the heat from below in some, cases can absorb only certain parts of the field, if it consists for example of a relatively unfavorable material, is slotted or does not have an adequate thickness for a full skin depth, so that the remaining proportion is transferred into the base of the receptacle.
A further possibility which can be used with the solution according to the invention is that of using receptacles closed by lids, the lids consisting of an electrically conducting material, for example metal, and it being possible for them to be designed in a way similar for example to yoghurt pots. In this case, the upper edge of the receptacle and/or the lid may be designed in such a way that these receptacles can be placed or hung in a frame-shaped element, and the heating can consequently be carried out in a corresponding device. In this case, the latching of the frame-shaped element and the size of the receptacles can be optimally adapted to the surface area available for the heating, so that maximum space utilization is possible.
The steam to be used for the heating, may, however, also be generated by heating water, which can be stored for example in a porous body, which may consist at least partially of an electrically conducting material. Various sintered metal bodies with adequately large pores and pore volumes may be used for example for this purpose. Such a porous body may be designed as a ring which has a greater diameter than the respective receptacles used and into which such receptacles can be placed, here again appropriate positioning with respect to at least one induction heating coil having to be taken into consideration. The porous bodies have, furthermore, the advantage that a quite specific amount of water can be absorbed and stored in them without any great expenditure. The porous bodies may, however, also consist of some other material as a pure water reservoir and the evaporation may be achieved by utilizing heat conduction, convection and/or radiation to the bodies.
A further possible way in which water can be provided for the evaporation is that of using, for example, structures in the form of small tubes, in the cavities of which the water can be temporarily stored. The steam formed can then possibly diffuse through permeable membranes or emerge from openings. In principle there is also the possibility of keeping the water available in small exchangeable storage bottles, which are preferably arranged with their opening downward during the heating. In this way, water or externally generated steam can be filled into or passed through small tubes, in which holes are possibly temporarily closed by foils or films, advantageously of semipermeable material.
The water may, however, also be temporarily stored in receptacles consisting completely of foils or films Pure plastic films, plastic films and metal foils together or metal-coated films may be used. Such a receptacle may be a disposable product. The water or the already formed steam can emerge after manual opening of the film or foil or, in a temperature-dependent manner, after exceeding a certain temperature during heating.
For the case in which receptacles which are suitable by their design and material selection for being heated inductively are used for the meals to be heated, there is in the simplest form the possibility of keeping the water required for the steam generation available directly in such a receptacle before the heating of the meals, in which case possibly separated receptacle regions or at least partially flexible receptacles of/with film or foil material can be used for this purpose, said receptacle regions or receptacles bursting open, releasing openings or possibly disintegrating under increasing steam pressure.
If the steam which is used for heating the meals is generated directly in the receptacles, it is expedient to cover the receptacle or a region around this receptacle with a lid-like formation, preferably in a gastight manner, so that a self-contained environment is produced. This solution is suitable in particular when using already loaded, relatively large trays or similar such formations on which receptacles with various items of food or drinks can be arranged, not all of which are to be heated. This has the advantage that such a tray can be automatically pre-loaded and, following heating, no additional efforts have to be expended to bring the various items of food and drinks together to form a complete portion.
It is also important that the amount of steam required for the heating is influenced by providing a meal-specific amount of water, so that the quality of the items of food heated is not influenced negatively but positively.
By contrast with the tray solution already described, the heating of prepared meals accommodated in receptacles may also be achieved by slight modification of conventional devices, such as they are for example the circulating air ovens used up until now on aircraft. In this case, such heating devices are dimensioned at least virtually identically and are fitted out with appropriate connection possibilities for electrical energy.
The elements for controlling and operating such a heating device for meals should be arranged on a removable panel or be integrated in a door, in order to minimize the space required. Other elements, such as for example the electronics present in any case on aircraft, may also be used for the control.
The combination of heating and alternating cooling in such an oven or meal transporting cart is advantageous. Combined units in which the items of food and drinks are first cooled and only heated up immediately before consumption can be used. As a result, storage space can be saved. For the cooling, it is possible to resort to conventional principles.
The receptacles filled with meals are introduced into additional push-in containers and, preferably by their own shape and dimensioning, positioned with respect to induction heating coils in such a way that optimum heating is achieved. Such induction heating coils are arranged at least in the base of such a push-in container, it also being possible for achieving an appropriate heat from above for corresponding induction heating coils to be integrated in the upper part of such a push-in container. For the inductive heating, the base and top of such a push-in container may be provided, again as already described, with sheet-like elements which can be heated by induction. Furthermore, the steam generation can, as likewise already described, also again be achieved by virtually exclusively inductive heating in such a push-in container, which in that case is closed.
Such push-in containers may also be used for the heating/regeneration of, for example, meals already covered on plates, it also being required here for the dimensioning of the containers to conform to the customary standard sizes.
Since, as already mentioned, standard dimensions have to be observed for the heating device (ovens) and meal transporting carts with which the heated meals are transported from the heating to the place where they are actually consumed, it is expedient to dimension the receptacles used for heating in an appropriately space-optimized manner, so that the space available for the heating and transport can be utilized to the maximum. For this purpose, the surface area respectively available on one level should be almost exactly an integral multiple of the surface area requirement of individual containers, whereby an increase in the surface area utilization and capacity of up to 50% can be achieved.
Such meal transporting carts are generally insulated with respect to the outside, in order to at least hinder undesired cooling or heating of the corresponding meals contained in them. For this purpose, conventional insulating materials which achieve limited K values and bring about an increase in mass are used. In particular on aircraft, on which such meal transporting carts are usually referred to as trolleys, it is extremely favorable, however, to achieve the insulation by creating a vacuum, or at least a partial vacuum, in the walls of such a trolley. Since on aircraft the corresponding negative pressure is available above certain, altitudes and such a negative-pressure generating system is present in any case for the toilet systems customarily to be used there, at least a partial vacuum can be created by a corresponding connection, or can be readily renewed again as and when required. In this case, not only can the walls then be partially evacuated, but also the entire content of such a trolley can be at least partially evacuated if there is appropriate sealing. Furthermore, the vacuum or partial vacuum may be favorably created by means of ejectors. In a corresponding way, the push-in containers may also be subjected to a vacuum on their own or additionally.
The heating of the meals may also take place, however, directly in such a meal transporting cart, this possibility preferably being able to be used whenever already pre-loaded trays on which various items of food and drinks to be heated or kept cold are arranged.
In this case it is possible to use planar, tongue-like elements which are connected to a high-voltage frequency generator via lines and into which induction heating coils are integrated. These tongue-like elements can then be introduced into such a trolley through opened doors of at least one side, or slots formed for example in the side walls, the induction heating coils, possibly in conjunction with the planar sheet-like elements which are to be inductively heated, being arranged in a way corresponding to the positioning of the receptacles in which the meals to be heated are accommodated, so that only heated where necessary and the other items of food and drinks remain virtually uninfluenced.
Profilings, with the aid of which the positioning or meals to be heated is facilitated, may be formed on the tongue-shaped elements.
For detecting the activation of the induction heating coils and also for measuring the temperature, the chance in inductance, the impedance, the capacitance and/or the change in frequency may be taken into consideration. The corresponding sensing and/or current or voltage measurement then preferably takes place by means of the electronics for the control of the frequency generator.
Since a plurality of induction heating coils, which are not to be, or do not always have to be, used simultaneously for the healing of meals, may be arranged in the tongue-shaped elements, it is favorable to provide a so-called xe2x80x9cpot detectionxe2x80x9d, which may take place for example in the form of a measurement of inductance, impedance or capacitance.
Since the ohmic resistance and inductance of many materials changes temperature-dependently, these properties can be used for contactless temperature monitoring.
Such tongue-like elements in conjunction with the corresponding high-frequency generator may be permanently installed in buildings, but also in vehicles, so that a correspondingly pre-loaded meal transporting cart be transported to such a device, the desired meals be appropriately heated with the latter and subsequently be transported to the respective final consumer. As a result, a considerable rationalizing effect can be achieved for a wide variety of application areas.
In this case, these tongue-like elements can of course also be arranged on a plurality of levels appropriately above and below the receptacles to be heated and for example metal sheets possibly in between.
It is advantageous in any case to arrange the induction coils in removable carriers, so that they can be removed from the oven for cleaning.
The electronically controlled high-frequency generator is operated in a frequency range above the audible limit for man, normally in the range between 20 and 50 kHz, it being the case in the aviation application in particular that the electronics are preferably of a redundant configuration, appropriate filters are used on the electrical energy supply and an input of preferably 400 Hz is used there. Both the electronics and the generator should be arranged in a separate housing, which is preferably also of a suitably shielded design, in order to avoid negative influences on the surroundings.
It is favorable to arrange additional components, in particular at the top and bottom in an oven, as ferrites, in order to counteract heating of the housing and to influence the electromagnetic field (EMC protective function).
Receptacles which, in a sandwich type of construction, on the outside of a plastic, including of an foamed form (improvement of the insulation), possibly provided with a fiber reinforcement, and on the inside with metal inserts or coatings, can be used as receptacles for heating. In this case, it is particularly favorable that also the edge is enclosed above with a metallic material, so that no direct contact can occur between plastic and food, These receptacles may also be closed by a lid, likewise of such a composite material, correspondingly metallically coated or exclusively of metal.
The lid or an insert may also be provided with a separate coil, which serves exclusively for the evaporation of water. The lid or an insert of receptacles or other elements may be designed in such a way that food (for example bread), drinks or other items (hand towels) can likewise be heated on it.
The plastic and metal composite may be permanently bonded for example by lamination, injection molding, adhesive bonding. There is also the possibility, however, of using a corresponding metal insert, which can be inserted into the plastic in a shape-adapted manner. In this case, the metal insert can be removed from the plastic, stored separately and cleaned relatively easily in this form.
A further advantageous refinement of the invention can be achieved by it being possible for the various items of food, and in this case in particular also the items of food to be heated, to be detected on the basis of the receptacles used by sensors suitable for this purpose, so that heating does not take place at all or a heating regime which is optimum for the respective item of food can be selected. For this purpose, receptacles of different colors or provided with a corresponding coding may be used, so that the temperatures and heating times can be optimally selected.
It has advantageous effects if at least a partial vacuum is created by internal pressure reduction in the receptacles filled with prepared meals. As a result, it is even possible in many cases to dispense with cooling, even for several days, and for the oxidation of the food to be at least hindered. For this purpose, the receptacles may have additional connections with valves, which can be connected to a negative-pressure-generating unit (for example an ejector).
In the case of the invention, temperature measurements may also be carried out to control or regulate the respective eddy currents, and accordingly also the corresponding heating. The temperature measurement in conjunction with a time measurement ensures that the respective items of food are heated to the correct temperature and undesired impairment of the items of food by overheating can be avoided.
With the temperature measurement, the power electronics for controlling the frequency generator or the activation of individual induction coils is influenced in particular.
For this purpose, the electrical or magnetic coil parameters can be measured, for example via auxiliary coils. There is also the possibility, however, of measuring the temperature via sensors. In this case, such sensors which are contacted directly with the respective receptacles, or possibly a frame-shaped element into which the receptacles can be inserted, may be used. Pin-shaped or spring-shaped temperature measuring contacts, for example measuring on a semiconductor basis, may be used for this purpose. Pins may engage in openings in the receptacles for the measurement.
The already mentioned push-in containers, or possibly also appropriate receptacles for meals, should advantageously be differently designed with respect to width and length, so that they can only ever be introduced in a certain position into a heating device or a meal transporting cart. Their outer shaping should be such that they can also be stacked securely and with the least space requirement. Furthermore, there should be edge regions which make it possible for push-in containers or receptacles arranged in a series or next to one another to be temporarily connected to one another positively, for example by hooking.
The push-in containers should be produced essentially from high-temperature-resistant plastics, for example polyhydroxybutyrate-compostable), which have at their lateral edges electrodes which serve as contacts. Via these electrodes, the high-frequency electrical energy can pass from the outside to the induction heating coils arranged in the base or in the top region, via connecting lines likewise integrated there. These electrodes may at the same time serve for keeping the push-in containers in connection with counterelectrodes arranged in the housing of an oven.
The induction heating coils are advantageously arranged and connected both in series and in parallel.
With the invention it is possible to reduce the time for adequate and optimum heating of meals in the catering industry by at least 50% and also to achieve a considerable increase in efficiency.
The invention is to be described in more detail below on the basis of exemplary embodiments.